Sliding member

ABSTRACT

A sliding member includes a back-metal layer including an Fe alloy and a sliding layer including a copper alloy including 0.5 to 12 mass % of Sn and the balance of Cu and inevitable impurities. A cross-sectional structure of the sliding layer includes first copper alloy grains in contact with a bonding surface and second copper alloy grains not in contact with the bonding surface. The first and second grains have an average grain size D1 and D2 respectively. D1 is 30 to 80 μm; and D1/D2=0.1 to 0.3. In the cross-sectional structure, the second grains includes third grains that includes internal grains therein that are not in contact with a grain boundary of the third grains. A total area S1 of the third grains and a total area of the second copper alloy grains S2 satisfy: S0/S2=0.25 to 0.80.

TECHNICAL FIELD

The present invention relates to a sliding member, for example for abearing used in internal combustion engines or automatic transmissions,or a bearing used in various machines. Specifically, the presentinvention relates to a sliding member including a sliding layer on aback-metal layer.

BACKGROUND ART

A cylindrical or semi-cylindrical sliding bearing formed of a slidingmember including a sliding layer of a copper alloy and a steelback-metal layer has been used for a bearing device of an internalcombustion engine, an automatic transmission, or the like. For example,JP 6-322462A and JP 2006-22896A describe a sliding member including asliding layer made of a copper alloy. In such a sliding member, thesliding layer made of a copper alloy achieves seizure resistance andwear resistance as well as sliding properties, while the back-metallayer functions as a support of the copper alloy and imparts strength tothe sliding member. JP 6-322462A and JP 2006-22896A describe a slidingmember formed through a sintering process, and JP 9-100882A (paragraph[0031]) describes a sliding member including a sliding layer made of acopper alloy and formed on an inner surface of a cylindrical back-metallayer by a casting process.

SUMMARY OF THE INVENTION

When the internal combustion engine or the automatic transmission startsto operate, a sliding surface of the sliding member is in direct contactwith a counter shaft member. Thus, when a bearing device starts theoperation and slides against the counter shaft member, the slidingsurface of the sliding member is in direct contact with the countershaft member. During a period from the moment when the counter shaftmember starts to move (i.e. rotate) until the time when the slidingmember and the counter shaft member are in a dynamic friction state(i.e. a state in which the sliding surface of the sliding member slidesagainst the counter shaft member), a large external force is appliedfrom the counter shaft member to the sliding layer of the sliding memberin a moving direction (or a sliding direction) of the counter shaftmember, thereby causing elastic deformation of the sliding layer. Whenthe elastic deformation of the sliding layer becomes larger under such asituation, a conventional sliding member which includes a sliding layerof a copper alloy on a back-metal layer is, in some cases, subjected toshear failure at an interface between the sliding layer and theback-metal layer or damages such as cracks on a sliding surface.

An object of the present invention is to provide a sliding member thatis less likely to be subjected to shear failure between the slidinglayer and the back-metal layer and has improved bonding between asliding layer and a back-metal layer and damages such as cracks on thesliding surface than in a conventional sliding member

According to an aspect of the present invention, provided is a slidingmember including: a back-metal layer having a back surface and a bondingsurface; and a sliding layer on the bonding surface of the back-metallayer. The back-metal layer includes an Fe alloy, and the sliding layerincludes a copper alloy including 0.5 to 12 mass % of Sn and the balanceof Cu and inevitable impurities. The sliding layer has a cross-sectionalstructure perpendicular to a sliding surface of the sliding layer. Thecross-sectional structure includes first copper alloy grains that are incontact with the bonding surface of the back-metal layer and secondcopper alloy grains that are not in contact with the bonding surface.The first copper alloy grains has an average grain size D1 and thesecond copper alloy grains has an average grain size D2, and D1 and D2satisfy the following relations:

D1 is 30 to 80 μm; and

D1/D2=0.1 to 0.3.

In the cross-sectional structure, the second copper alloy grainscomprise third grains that include internal grains therein that are notin contact with a grain boundary of the third grains. A total area S0 ofthe third grains and a total area S2 of the second copper alloy grainssatisfy the following relation:

S0/S2=0.25 to 0.80.

According to an embodiment of the present invention, the second copperalloy grains are preferably columnar crystals. According to anembodiment of the present invention, the first copper alloy grains arealso preferably columnar crystals.

According to an embodiment of the present invention, the average grainsize D1 of the first copper alloy grains is preferably 40 to 80 μm.

According to an embodiment of the present invention, an averagethickness T1 of a group of the first copper alloy grains is preferably 3to 8% of a thickness T of the sliding layer.

According to an embodiment of the present invention, the thickness T ofthe sliding layer is preferably 0.4 to 2.0 mm.

According to an embodiment of the present invention, the copper alloy ofthe sliding layer preferably further includes one or more elementsselected from 0.01 to 0.2 mass % of P, 0.1 to 15 mass % of Ni, 0.5 to 10mass % of Fe, 0.01 to 5 mass % of Al, 0.01 to 5 mass % of Si, 0.1 to 5mass % of Mn, 0.1 to 10 mass % of Zn, 0.1 to 5 mass % of Sb, 0.1 to 5mass % of In, 0.1 to 5 mass % of Ag, 0.5 to 25 mass % of Pb, and 0.5 to20 mass % of Bi.

According to an embodiment of the present invention, the back-metallayer preferably includes 0.07 to 0.35 mass % of C, not more than 0.4mass % of Si, not more than 1 mass % of Mn, not more than 0.04 mass % ofP, not more than 0.05 mass % of S, and the balance of Fe and inevitableimpurities.

In the sliding member of the present invention, shear failure is lesslikely to occur at a bonding interface between the sliding layer and theback-metal layer, leading to stronger bonding between the back-metallayer and the sliding layer and less occurrence of damages such ascracks on the sliding surface.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a cross section of an embodiment of asliding member according to the present invention.

FIG. 2 is a schematic diagram of a cross-sectional structure of asliding layer shown in FIG. 1.

FIG. 3 is a perspective view of an embodiment of the sliding memberaccording to the present invention.

FIG. 4 is a schematic diagram of a copper alloy layer structure aftercasting of an embodiment of a process of manufacturing a sliding memberaccording to the present invention.

FIG. 5 shows an example of manufacturing condition of the sliding memberaccording to the invention.

DESCRIPTION OF THE EMBODIMENTS

In general, a sliding member includes a sliding layer including a copperalloy and formed on one surface of a back-metal layer. The back-metallayer typically includes hypoeutectoid steel including 0.07 to 0.35 mass% of carbon, stainless steel, or the like. The copper alloy of thesliding layer typically includes 1 to 12% of Sn, and has a structureincluding a large number of copper alloy grains.

In the conventional sliding member, the copper alloy grains have anapproximately uniform shape and grain size throughout a thicknessdirection of the sliding layer. Thus, the sliding layer hasapproximately uniform deformation resistance against an external forcethroughout the thickness direction of the sliding layer. In such aconventional sliding member, when operation of a bearing device startsand a sliding surface of the sliding member slides against a countershaft member while it is in direct contact with the counter shaftmember, a large external force is applied from the counter shaft memberto the sliding layer in a moving direction (sliding direction) of thecounter shaft member during a period from the moment when the movement(rotation) of the counter shaft member starts until the time when thesliding member and the counter shaft member are shifted to a dynamicfriction state. Thus, elastic deformation of the sliding layer iscaused. Since the back-metal layer has high strength and highdeformation resistance as compared with the copper alloy of the slidinglayer, a large difference in the elastic deformation generates betweenthe back-metal layer and the sliding layer at their interface when anamount of elastic deformation of the sliding layer becomes larger. Thus,shear failure is more likely to generate between the back-metal layerand the sliding layer. Also, a large amount of elastic deformation ofthe sliding layer causes cracking due to fatigue or the like at a grainboundary in the sliding layer, and cracking is more likely to occur onthe surface of the sliding layer. The present invention addresses such aproblem of the conventional sliding member.

An embodiment of a sliding member 1 according to the present inventionis described below with reference to FIGS. 1 to 3. FIG. 3 is aperspective view showing an embodiment of the sliding member 1 accordingto the present invention. The sliding member 1 has a cylindrical shape,and includes a back-metal layer 2 on an outer side and a sliding layer 3including a copper alloy 4 on an inner side. The sliding member 1 mayhave other shapes such as a semi-cylindrical shape or a partiallycylindrical shape.

FIG. 1 is a schematic diagram showing a cross section of the slidingmember 1 of the present invention. The back-metal layer 2 has a backsurface 22 and a bonding surface 21, and the sliding layer 3 is formedon the bonding surface 21. The sliding layer 3 has an interface with theback-metal layer 2, and has a sliding surface 31 on a side opposite tothe interface. A material of the back-metal layer 2 is not particularlylimited, but may be, for example, an Fe alloy including 0.07 to 0.35mass % of carbon, stainless steel, or the like. For example, theback-metal layer 2 may have a composition including 0.07 to 0.35 mass %of carbon, and one or more elements of not more than 0.4 mass % of Si,not more than 1 mass % of Mn, not more than 0.04 mass % of P, and notmore than 0.05 mass % of S, and the balance of Fe and inevitableimpurities. In some cases, a small amount of Cu element of the copperalloy diffuses and solid-solved in a ferrite phase in the vicinity ofthe bonding surface 21 of the back-metal layer 2 in a casting process(described later). The back-metal layer 2 may further include suchcopper element other than the above composition.

The copper alloy 4 of the sliding layer 3 may have a composition, forexample, including 0.5 to 12 mass % of Sn and the balance of Cu andinevitable impurities. Sn is an element that increases strength of thecopper alloy. If the copper alloy includes less amount of Sn than thelower limit value, the effect of Sn is insufficient. If the amount of Snincluded in the copper alloy is more than the upper limit value, thecopper alloy becomes brittle.

The copper alloy 4 may, for example, include 0.5 to 12 mass % of Sn, andone or more elements selected from 0.01 to 0.2 mass % of P, 0.1 to 15mass % of Ni, 0.5 to 10 mass % of Fe, 0.01 to 5 mass % of Al, 0.01 to 5mass % of Si, 0.1 to 5 mass % of Mn, 0.1 to 10 mass % of Zn, 0.1 to 5mass % of Sb, 0.1 to 5 mass % of In, 0.1 to 5 mass % of Ag, 0.5 to 25mass % of Pb, 0.5 to 20 mass % of Bi, and the balance of Cu andinevitable impurities. When the copper alloy 4 includes two or more ofthese selected elements, a total amount of the elements is preferablynot more than 40 mass %.

P, Ni, Fe, Al, Si, Mn, Zn, Sb, In and Ag increase strength of the copperalloy 4. If the amounts of these elements included in the copper alloy 4are less than the lower limit value, the effect of these elements isinsufficient, and if the amounts of these elements included in thecopper alloy 4 are more than the upper limit value, the copper alloy 4becomes brittle. When a copper alloy including the amounts of Al and Znmore than the upper limit value is formed by a casting process on aback-metal layer of an Fe alloy, a layered brittle reaction phase (aphase including Fe as a main component, Cu of the copper alloy, and Alor Zn) is formed on the entire bonding surface of the back-metal layerbonded to the copper alloy. In a case where such a brittle reactionphase is formed on the bonding surface of the back-metal layer, anexternal force (i.e., a large external force generated at the start ofthe sliding described above) applied to the copper alloy of the slidinglayer of the sliding member will generate cracking in the brittlereaction phase and thus shear failure is more likely to occur betweenthe sliding layer and the back-metal layer. Pb and Bi are elements thatincrease lubricating properties of the copper alloy 4. If the amounts ofPb and Bi included in the copper alloy 4 are less than the lower limitvalue, the effect of Pb and Bi is insufficient, and if the amounts of Pband Bi included in the copper alloy 4 are more than the upper limitvalue, the copper alloy 4 is brittle. Sn and the selected elements ofthe copper alloy 4 are almost uniformly included in the entire copperalloy 4 of the sliding layer 3, and there is no difference inconcentration of these elements between (later defined) interface grains41 and non-interface grains 42.

FIG. 2 shows a cross-sectional structure perpendicular to the slidingsurface 31 of the sliding layer 3 shown in FIG. 1 (hereinafter, thecross section perpendicular to the sliding surface 31 of the slidinglayer 3 is simply referred to as “cross section”). The structure of thecopper alloy 4 constituting the sliding layer 3 includes a large numberof copper alloy grains. The copper alloy grains include grains 41 thatare in contact with the bonding surface 21 of the back-metal layer 2(hereinafter referred to as “interface grains”) and grains 42 that arenot in contact with the bonding surface 21 of the back-metal layer 2(hereinafter referred to as “non-interface grains”). In the presentinvention, an average grain size D1 of the interface grains 41 is 10 to30% of an average grain size D2 of the non-interface grains 42(D1/D2=0.1 to 0.3). Thus, in a region composed of the interface grains41 (hereinafter referred to as “interface grain group 41G”), deformationresistance to external force is larger than in a region composed of thenon-interface grains 42 (hereinafter referred to as “non-interface graingroup 42G”) due to Hall-Petch effect.

In the cross-sectional structure of the copper alloy 4 constituting thesliding layer 3, the non-interface grains 42 include copper alloy grains(hereinafter referred to as “internal-crystal-inclusive grains 43”)including other grains (hereinafter referred to as “internal crystals44”) that are apart from a grain boundary of theinternal-crystal-inclusive grains 43. That is, in cross-sectional viewof the copper alloy 4, the internal crystals 44 are grains that arepresent within the internal-crystal-inclusive grains 43 and are not incontact with the grain boundary of the internal-crystal-inclusive grains43. An area ratio of the internal-crystal-inclusive grains 43 to thenon-interface grain group 42G is 25 to 80%. That is, when S2 representsa total area of the non-interface grains 42, and S0 (hereinafter, S0includes an area of the internal crystals 44) represents a total area ofthe internal-crystal-inclusive grains 43, a ratio S0/S2 is 0.25 to 0.80.

In the present invention, the non-interface grains 42 are preferablycolumnar crystals. As described below, it is advantageous for formingthe internal-crystal-inclusive grains 43 that the columnar crystals growin different directions. However, in that case, at least some of thenon-interface grains 42 may not necessarily be columnar crystals.Furthermore, in the present invention, the interface grains 41 are alsopreferably columnar crystals. This is because the interface grains 41composed of columnar crystals are advantageous for growth of thenon-interface grains 42 in different directions. Also in that case, atleast some of the interface grains 41 may not necessarily be columnarcrystals. FIG. 2 shows a case where the interface grains 41 and thenon-interface grains 42 are both columnar crystals.

With this configuration, following effects are obtained. Even if whenoperation of a bearing device starts and an external force is appliedfrom a counter shaft member to the sliding layer 3 to cause elasticdeformation of the sliding layer 3 during a period from the moment whenrotational movement of the counter shaft member starts until the timewhen the sliding member and the counter shaft member are shifted to thedynamic friction state, a large amount of elastic deformation occurs inthe non-interface grain group 42G having relatively low deformationresistance, but the external force is less likely to be applied to theinterface grain group 41G. Furthermore, due to high deformationresistance and a small amount of elastic deformation of the interfacegrain group 41G, a difference in the amount of elastic deformationbetween the back-metal layer 2 and the copper alloy 4 is small on thebonding surface 21, and thus shear failure is less likely to occur onthe bonding surface 21, leading to stronger bonding between theback-metal layer 2 and the copper alloy 4 of the sliding layer 3.

If the average grain size D1 of the interface grains 41 is more than 30%of the average grain size D2 of the non-interface grains 42, differencein deformation resistance between the interface grain group 41G and thenon-interface grain group 42G is small, and thus the above effect isinsufficient. On the other hand, if the average grain size D1 of theinterface grains 41 is less than 10% of the average grain size D2 of thenon-interface grains 42, difference in deformation resistance betweenthe interface grain group 41G and the non-interface grain group 42G isexcessively large, and thus cracking may occur between the interfacegrain group 41G and the non-interface grain group 42G when an externalforce is applied to the sliding layer.

The average grain size D1 of the interface grains 41 is preferably 30 to80 μm, and more preferably 40 to 80 μm. Since the interface grains 41have a smaller average grain size D1, the interface grain group 41G incontact with the bonding surface 21 of the back-metal layer 2 has higherdeformation resistance. However, if the interface grains 41 have anaverage grain size D1 of less than 30 μm, ductility becomes excessivelylow, and thus cracking (or fatigue) may occur at a grain boundary when alarge external force is applied to the sliding layer 3. On the otherhand, if the average grain size D1 exceeds the upper limit value, theinterface grain group 41G in contact with the bonding surface 21 of theback-metal layer 2 has small deformation resistance, and the effect ofpreventing the occurrence of shear failure is less likely to beachieved.

In addition to the above effect, according to the present invention, theinternal-crystal-inclusive grains 43 among the non-interface grains 42prevent excessive elastic deformation of the entire non-interface graingroup 42G, and damages such as cracks on the sliding surface is lesslikely to occur. This is because the internal crystals 44 and crystalssurrounding the internal crystals 44 have different crystal orientationsin the internal-crystal-inclusive grains 43, and thus the internalcrystals 44 serve as deformation resistance, thereby preventing theexcessive elastic deformation in the sliding direction despite a largegrain size of the non-interface grain group 42G.

A ratio (S0/S2) of the total area S0 of the internal-crystal-inclusivegrains 43 to the area S2 of the non-interface grain group 42G ispreferably 25 to 80%. The ratio (S0/S2) is more preferably 40 to 80%. Ifthe ratio is less than 25%, the elastic deformation cannot be prevented,and the effect of preventing the occurrence of damages such as cracks onthe sliding surface is less likely to be achieved. When the ratio ise.g. 40%, the effect of preventing the elastic deformation is enhanced.On the other hand, if the area ratio is more than 80%, difference indeformation resistance between the interface grain group 41G and thenon-interface grain group 42G is small, and the above effect ofpreventing the occurrence of shear is insufficient.

Next, a method of measuring the average grain size D1 of the interfacegrains 41 and the average grain size D2 of the non-interface grains 42will be described.

First, a plurality of portions (e.g., 5 portions) of a cross section cutin a thickness direction of the sliding member 1 (i.e., a directionperpendicular to the sliding surface 31) are observed with use of anFE-SEM (field emission scanning electron microscope) (e.g., JIB-4600Fmanufactured by JEOL Ltd.) including an EBSD (electron backscatterdiffraction) analysis system. Then, a crystal orientation angle of thecopper alloy is analyzed, and images (e.g., at a magnification of 200times) showing a grain boundary of the copper alloy grains are obtained.The grain boundary of the copper alloy grains is determined by aninclination angle of not less than 15°.

Subsequently, the interface grains 41 and the non-interface grains 42 inthe images are distinguished from each other with use of a general imageanalysis method (e.g., analysis software: Image-Pro Plus (Version 4.5)manufactured by Planetron, Inc.), and an area of each of the interfacegrains 41 is measured. Then, a diameter of a circle having an area equalto the area of each of the interface grains 41 (equivalent circlediameter) is calculated, and he average grain size D1 is calculated. Ina similar manner, the average grain size D2 of the non-interface grains42 is obtained. The magnification of the images is not limited to 200times, and may be changed to other magnifications.

Next, a method of measuring the area ratio (S0/S2) will be described.

First, as the measuring of the average grain size D1 of the interfacegrains 41 and the average grain size D2 of the non-interface grains 42,a plurality of portions (e.g., 5 portions) of a cross section cut in thethickness direction of the sliding member 1 (i.e., the directionperpendicular to the sliding surface 31) are observed with use of anFE-SEM (field emission scanning electron microscope) (e.g., JIB-4600Fmanufactured by JEOL Ltd.) including an EBSD (electron backscatterdiffraction) analysis system. Then, a crystal orientation angle of thecopper alloy is analyzed, and images (e.g., at a magnification of 200times) showing a grain boundary of the copper alloy grains are obtained.The grain boundary of the copper alloy grains is determined by aninclination angle of not less than 15°.

Subsequently, with use of a general image analysis method (e.g.,analysis software: Image-Pro Plus (Version 4.5) manufactured byPlanetron, Inc.) for the images, the interface grains 41, thenon-interface grains 42 that are not in contact with the bonding surface21 of the back-metal layer 2, and the internal-crystal-inclusive grains43 including the internal crystals 44 in the non-interface grains 42 aredistinguished from each other, and then a total area (S2) of thenon-interface grains 42 and an area (S0) of theinternal-crystal-inclusive grains 43 are measured. From the total area(S2) and the area (S0), the area ratio S0/S2 is obtained. Themagnification of the images is not limited to 200 times, and may bechanged to other magnifications. In some cases, a granular intermetalliccompound including Sn, the selected elements, or the like mentionedabove is formed (deposited) at the grain boundary of the grains in thecopper alloy 4. As well, a granular Bi or Pb phase e is formed(deposited) in some cases at the grain boundary of the copper alloygrains. However, the intermetallic compound grains, the Bi phase, the Pbphase, and the like are not taken into consideration in the measurementof the average grain sizes D1 and D2.

An average thickness T1 of the interface grain group 41G is defined asan average length of the interface grain group 41G in a direction fromthe bonding surface 21 toward the sliding surface 31. The averagethickness T1 of the interface grain group 41G is preferably 3 to 8% of athickness T of the sliding layer (T1/T=0.03 to 0.08).

If the average thickness T1 of the interface grain group 41G is lessthan the lower limit value, a large external force applied to thesurface of the sliding layer 3 may be transmitted to a portion of theinterface grain group 41G in the vicinity of the bonding surface 21 ofthe back-metal layer 2. If the average thickness T1 of the interfacegrain group 41G is more than the upper limit value, the non-interfacegrain group 42G has an excessively small thickness ratio, and thus theeffect of mitigating the external force may be insufficient. Thethickness T of the sliding layer 3 is preferably 0.3 to 3.0 mm, and morepreferably 0.3 to 2.0 mm.

The average thickness of the interface grain group 41G can be obtainedfrom the images with use of the image analysis method. An average lengthis measured thereby in the thickness direction from the bonding surface21 of the back-metal layer 2 to a boundary (line) between the interfacegrain group 41G and the non-interface grain group 42G. The thickness Tof the sliding layer 3 can also be confirmed, from the images andmeasuring with use of the image analysis method, as an average length inthe thickness direction from the bonding surface 21 of the back-metallayer 2 to the sliding surface 31.

The sliding member of the present invention may include, on the surfaceof the sliding layer and/or the surface of back-metal layer, a coatinglayer of Sn, Bi or Pb or an alloys based on these metals, or a coatinglayer of a synthetic resin or based on a synthetic resin. (In this case,the surface of the sliding layer is herein referred to as “slidingsurface”).

A method of producing a sliding member according to the presentembodiment will be described below.

First, a cylindrical member of an Fe alloy to be formed into aback-metal layer and a molten copper alloy to be formed into a slidinglayer are prepared. The molten copper alloy is maintained at atemperature higher by approximately 50° C. to 150° C. than a meltingpoint (liquidus temperature) of the copper alloy. The cylindrical memberis heated and maintained at a temperature in the range fromapproximately the same temperature as the temperature of the moltencopper alloy to be cast (poured) into the cylindrical member to atemperature lower by 50° C. than the temperature of the molten copperalloy. In order to prevent oxidation of the cylindrical member duringheating, flux may be applied in advance to at least an inner surface (tobe bonded with the copper alloy of the sliding layer) or the entiresurface of the cylindrical member.

The prepared cylindrical member is mounted to a centrifugal castingdevice, and the molten copper alloy at the above temperature is pouredonto the inner surface of the cylindrical member, and then thecylindrical member is rotated around an axis of the cylindrical member.As shown in FIG. 5, immediately after the poring of the molten copperalloy, a rotational speed of the cylindrical member is set so that aratio (centrifugal force/gravity) between centrifugal force F andgravity G applied to the molten copper alloy is 0.8 to 0.9 (hereinafterreferred to as “initial gravity ratio F/Gi”), and after 1 to 5 seconds,the rotational speed is increased so that the ratio of the centrifugalforce to the gravity is 50.0 to 60.0 (hereinafter referred to as“reference gravity ratio F/Gs”). An amount of the molten copper alloypoured into the cylindrical member is not less than 10 times and notmore than 25 times an amount required to form a sliding layer having thethickness T of the sliding member. After the rotational speed of thecylindrical member is increased so that the gravity ratio reaches thereference gravity ratio F/Gs, cooling water is sprayed to an outerdiameter surface side of the cylindrical member to cool the cylindricalmember. Subsequently, while the molten copper alloy is at a temperatureof lower than the liquidus temperature and not lower than a solidustemperature, the rotational speed is changed so that the ratio of thecentrifugal force to the gravity is varied 3 to 10 times between 40 to70% of the reference gravity ratio F/Gs (hereinafter referred to as “endgravity ratio F/Ge”) and the reference gravity ratio F/Gs. When themolten copper alloy reaches a temperature of not higher thanapproximately 400° C., the rotation and cooling of the cylindricalmember are stopped, and the cylindrical member is removed from thecentrifugal casting device. The time from the start of the water coolinguntil the molten copper alloy is solidified is approximately 20 to 40seconds.

After the removal of the cylindrical member from the centrifugal castingdevice, the cylindrical member is allowed to be cooled to a roomtemperature in an air atmosphere or forced to be cooled to a roomtemperature by water cooling or the like. On the inner surface of thecylindrical member after cooling, a copper alloy layer is formed, andthe copper alloy layer has a thickness approximately 10 to 25 times thethickness T of the sliding layer required to constitute the slidingmember.

In a conventional manufacturing method of a sliding member through acentrifugal casting method, an amount of molten copper alloy poured intoa sliding cylindrical member is approximately 1.2 to 2 times an amountrequired to form a sliding layer 3 having the thickness T of the slidingmember. Thus, a copper alloy layer is formed on an inner surface side ofthe cylindrical member after cooling, and the copper alloy layer has athickness approximately 1.2 to 2 times the thickness T of the slidinglayer required to constitute the sliding member. The conventional copperalloy manufactured in this manner has a cross-sectional structure thatgenerally includes a chill crystal portion of the copper alloy on abonding interface of a back-metal layer, a columnar crystal portionabove the chill crystal portion, and an equiaxed crystal portion abovethe columnar crystal portion. The chill crystal portion is formed as athin film on the bonding interface of the back-metal layer (i.e.cylindrical member). For example, when a copper alloy layer having athickness of approximately 1 mm is formed, the chill crystal portion hasa thickness approximately 1% (0.01 mm) of the thickness of the copperalloy layer. Copper alloy grains of the chill crystal portion have anexcessively small average grain size (an average grain size of not morethan 1 μm), and thus the copper alloy grains of the chill crystalportion have high strength but are excessively brittle. Thus, duringsliding of a sliding surface of the sliding member with a counter shaftmember, shear failure is more likely to occur between the chill crystalportion and the bonding interface of the back-metal layer or between thechill crystal portion and the columnar crystal portion when an externalforce is applied to the copper alloy of the sliding layer.

In the conventional manufacturing method, simultaneously with thepouring of the molten copper alloy onto the inner surface of thecylindrical member (back metal), the molten copper alloy coming intocontact with the inner surface (bonding interface) of the cylindricalmember is cooled to form a large number of crystal nuclei, and themolten copper alloy is simultaneously solidified. In the vicinity of thebonding interface, copper alloy grains having extremely small grain sizeare simultaneously generated, and the copper alloy grains hinder mutualcrystal growth to form chill crystals. When the chill crystals areformed in the vicinity of the bonding surface 21, a concentration of Sn,which is a solute component, decreases toward the bonding surface 21 inthe copper alloy 4. This is because crystal nuclei are generated, due tothe rapid cooling of the molten copper alloy, in the molten copper alloyin the vicinity of the bonding interface as Cu (composed of Cu), and thecopper alloy grains simultaneously solidified in the vicinity of thebonding interface include the crystal nuclei as a main component andthus include a small amount of Sn which is a solute component.

In the method of the present invention, as described above, since alarge amount of molten copper alloy is poured onto the inner surface ofthe cylindrical member (back metal), the molten copper alloy coming intocontact with the inner surface (bonding interface) of the cylindricalmember is cooled slower than in the conventional method, therebyreducing the formation of crystal nuclei of the copper alloy in themolten copper alloy in contact with the bonding interface. Furthermore,since the molten copper alloy is cooled slower than in the conventionalmethod due to a large amount of poured molten copper alloy, part of themolten copper alloy including the crystal nuclei generated in thevicinity of the bonding interface flows from the bonding interface toanother portion before being solidified in the vicinity of the bondinginterface, following the rotating inner surface (bonding interface) ofthe cylindrical member and by centrifugal force applied to the moltencopper alloy. Thus, it is possible to prevent the simultaneousgeneration of a large number of crystal nuclei in the vicinity of theinner surface (bonding interface) and the simultaneous solidification ofthe molten copper alloy in the vicinity of the interface. Furthermore,the rotational speed is set, as shown in FIG. 5, so that the initialgravity ratio F/Gi is 0.8 to 0.9, the influence of the centrifugal forceon the crystal nuclei generated in the vicinity of the bonding interfaceis reduced, and the crystal nuclei are grown from the bonding interfacetoward a center of the axis of the cylindrical member to be columnarcrystals. Therefore, the copper alloy grains in contact with the bondinginterface of the back-metal layer can grow to have an average grain sizeof 30 μm to 80 μm. Subsequently, cooling water is sprayed to the outerdiameter surface side of the cylindrical member to cool the cylindricalmember, and while the molten copper alloy is at a temperature of lowerthan the liquidus temperature and not lower than the solidustemperature, the gravity ratio is varied 3 to 10 times between the endgravity ratio F/Ge and the reference gravity ratio F/Gs. By varying ofthe gravity ratio (i.e., rotational speed), gravity and varying inertialforce and centrifugal force are applied to the molten copper alloy inthe process of being solidified. Thus, due to unstable behavior of themolten copper alloy, growth of the columnar crystals in differentdirections causes growth of the columnar crystals intertwined with eachother. Therefore, internal-crystal-inclusive grains including internalcrystals are generated in cross-sectional view.

FIG. 4 is a schematic diagram showing a cross-sectional structure of acasted copper alloy layer after cooling. A copper alloy layer 5 includesa grain group 51 that is in contact with an inner surface of a copperalloy part and has an average grain size of 30 μm to 80 μm. The graingroup 51 is to be the interface grain group 41G of the sliding layer. Onthe grain group 51, a columnar crystal structure portion 53 having alarge average grain size is formed. In cross-sectional view of thecolumnar crystal structure portion 53, internal-crystal-inclusive grains55 are observed. On the columnar crystal structure portion 53, anequiaxed crystal structure portion 54 is formed. The equiaxed crystalstructure portion 54 is composed of granular equiaxed crystals having anaverage grain size smaller than the average grain size of the columnarcrystals. When T0 represents a thickness of the copper alloy layer, athickness T3 of the columnar crystal structure portion 53 isapproximately 15 to 35% of T0, and a thickness T4 of the equiaxedcrystal structure portion 54 is approximately 60 to 80% of T0.

Next, a portion on a surface (inner surface) side of the copper alloylayer is removed by cutting processing so that the copper alloy layerhas the thickness T. Thus, the sliding member 1 is manufactured. Thepart of the copper alloy layer after cooling is removed by the cuttingprocessing so that the thickness T0 is reduced by at least 90%, and thusthe equiaxed crystal structure portion 54 and part of the columnarcrystal structure portion 53 are removed. Accordingly, the non-interfacegrain group 42G of the sliding layer does not include the equiaxedcrystal structure portion 54, and includes a remaining part 52 of thecolumnar crystal structure portion composed of grains having a largeaverage grain size.

Thus, the average grain size D1 of the interface grain group 41G can be10 to 30% of the average grain size D2 of the non-interface grain group42G (D1/D2=0.1 to 0.3). In the manufacture of the sliding member 1, thecopper alloy layer may be subjected to annealing treatment, for example,at a temperature of 250° C. to 400° C. before or after the cuttingprocessing, in order to eliminate distortion of the copper alloy.

As described above, the amount of molten copper alloy poured into thecylindrical member is not less than 10 times and not more than 25 timesthe amount required to form a sliding layer having the thickness T ofthe sliding member. If the amount of poured molten copper alloy is lessthan 10 times the above amount, a chill crystal portion is formed asdescribed above and shear failure is more likely to occur. Even if nochill crystal portion is formed, when the copper alloy layer aftercooling is cut to have the thickness T of the sliding layer, the copperalloy layer may include an equiaxed crystal portion on the slidingsurface side. In such a case, the copper alloy grains that are not incontact with the bonding interface has a small average grain size, andthus the effect of the present invention is not obtained, and shearfailure may occur at the bonding interface. On the other hand, if theamount of poured molten copper alloy is more than 25 times the aboveamount, a structure similar to the copper alloy of the sliding layer ofthe present invention is obtained, but such a structure requires anexcessively large number of man-hours for the cutting and causes asliding member to be expensive, and is thus unfavorable.

If the initial gravity ratio F/Gi is less than 0.8, a small centrifugalforce causes an insufficient centrifugal separation effect, and gas,oxide or the like remains in the columnar crystal structure portion 53,leading to generation of cracks due to the pores. On the other hand, ifthe initial gravity ratio F/Gi is more than 0.9, columnar crystals areless likely to be generated at the bonding interface due to theinfluence of the centrifugal force, leading to the interface grain group41G having a small average grain D1. If the interface grain group 41Ghas an average grain size D1 of less than 30 μm, ductility isexcessively low. Thus, when a large external force is applied to thesliding layer 3, cracking (fatigue) occurs at the grain boundary.

If the end gravity ratio F/Ge is less than 40% of the reference gravityratio F/Gs, a small centrifugal force causes an insufficient centrifugalseparation effect, and gas remains in the columnar crystal structureportion 53, leading to generation of cracks due to the pores. If the endgravity ratio F/Ge is more than 70% of the reference gravity ratio F/Gs,the formation of the internal-crystal-inclusive grains 55 isinsufficient. Furthermore, if the gravity ratio is not varied betweenthe reference gravity ratio F/Gs and the end gravity ratio F/Ge, thecolumnar crystals can not grow in different directions and theinternal-crystal-inclusive grains 55 are not generated.

Furthermore, if the number of variations between the reference gravityratio F/Gs and the end gravity ratio F/Ge is less than 3, the aboveeffect is not obtained and cracking (or fatigue) may occur at the grainboundary. On the other hand, if the number of variations is more than10, a structure similar to the copper alloy of the sliding layer of thepresent invention is obtained, but such a structure requires anexcessively large number of man-hours and causes a sliding member to beexpensive, and is thus unfavorable.

EXAMPLE

Samples of Examples 1 to 15 of the sliding member according to thepresent invention and Comparative Examples 21 to 27 were produced in thefollowing manner. In the production of these samples, a back-metal layerwas formed of a cylindrical member of hypoeutectoid steel including 0.2mass % of carbon. In Examples 1 to 15, a copper alloy having apredetermined composition was melted to obtain a molten copper alloy andpoured onto the back-metal layer (inner surface of the cylindricalmember) by centrifugal casting process as described above. Aftercasting, the copper alloy layer was subjected to cutting processing, andthen subjected to annealing treatment at a temperature of 300° C. for 5hours to produce a sliding member. The samples were produced to have thesame size of an inner diameter of 80 mm, a thickness of 6 mm, and anaxial length of 80 mm to be subjected to a sliding test described later.

In a column “Thickness” in Table 1, a column “Sliding layer thickness T(mm)” shows the thickness T of the sliding layer of the samples. In acolumn “Manufacturing conditions” in Table 1, a column “Amount of pouredcopper alloy” shows a value M′/M, where M indicates the amount of moltencopper alloy required to form a sliding layer having the thickness shownin the column “Sliding layer thickness T (mm)” in Table 1, and M′indicates the amount of molten copper alloy poured onto the innersurface of the cylindrical member during centrifugal casting. InExamples 1 to 15, the amount of poured copper alloy was set to a valueof not less than 10 times the amount required to form a sliding layer.

The thickness of the copper alloy layer before the cutting processingwas approximately equal to a value obtained by multiplying the thicknessshown in the column “Sliding layer thickness T (mm)” by the value shownin the column “Amount of poured copper alloy”.

During the pouring of the molten copper alloy, a temperature of themolten copper alloy was higher by 50° C. to 150° C. than a melting point(liquidus temperature) of the copper alloy. In the column “Manufacturingconditions” in Table 1, a column “Molten copper alloy temperature (°C.)” shows a temperature difference of the temperature of the moltencopper alloy during the pouring of the molten copper alloy with respectto the liquidus temperature of the copper alloy. For example, “+50”indicates a temperature higher by 50° C. than the liquidus temperatureof the copper alloy.

During the pouring of the molten copper alloy, a temperature of theback-metal layer (cylindrical member) was set to be lower by 0 to 50° C.than the temperature of the molten copper alloy. In the column“Manufacturing conditions” in Table 1, a column “Back metal temperature(° C.)” shows a temperature difference of the temperature of theback-metal layer during the pouring of the molten copper alloy as withrespect to the temperature of the molten copper alloy.

The rotational speed of the cylindrical member was set so that the“initial gravity ratio F/Gi”, which was the ratio between thecentrifugal force and the gravity applied to the molten copper alloyimmediately after the pouring of the molten copper alloy, was 0.8 to0.9. After 5 seconds, the rotational speed of the cylindrical member wasset so that the “reference gravity ratio F/Gs” was 55. Then, after 10seconds, cooling water was sprayed to an outer diameter surface side ofthe cylindrical member to start cooling the cylindrical member.Subsequently, from a time when the molten copper alloy reached theliquidus temperature to a time when the molten copper alloy reached thesolidus temperature, the rotational speed of the cylindrical member waschanged so that the gravity ratio was varied 10 times between the “endgravity ratio F/Ge” (40 to 70% of the reference gravity ratio F/Gs) andthe reference gravity ratio F/Gs. In the column “Manufacturingconditions” in Table 1, a column “Initial gravity ratio (F/Gi)” showsthe “initial gravity ratio F/Gi” applied to the molten copper alloyimmediately after the pouring of the molten copper alloy of Examples asa ratio between the centrifugal force and the gravity applied to themolten copper alloy immediately after the pouring of the molten copperalloy. In the column “Manufacturing conditions” in Table 1, a column“End gravity ratio (F/Ge)” shows the “end gravity ratio F/Ge” ofExamples as a ratio (%) with respect to the reference gravity ratioF/Gs. For example “40%” indicates 40% of the reference gravity ratioF/Gs “55.0”.

After the pouring of the molten copper alloy into the cylindricalmember, cooling water was sprayed to the cylindrical member atapproximately 1 liter/second per 1 cm² to cool the cylindrical member toapproximately 400° C. in approximately 20 to approximately 40 seconds.

Among Comparative Examples 21 to 27 shown in Table 1, ComparativeExamples 21 to 26 were produced in the same manner as Examples accordingto the invention. The copper alloy layer was centrifugal cast and thenit was subjected to cutting process and annealing treatment at atemperature of 300° C. for 5 hours to produce a sliding member. InComparative Example 27, the sample was produced by a conventionalgeneral sintering process. A copper alloy powder was scattered on a flatplate composed of hypoeutectoid steel, and then subjected to firstsintering (810° C.), first rolling and second sintering (810° C.) toproduce a multilayer member. Then, the multilayer member was formed intoa cylindrical shape to produce a sliding member. The sliding members ofComparative Examples had the same size (an inner diameter of 80 mm, athickness of 6 mm, and an axial length of 80 mm) as the sliding membersof Examples according to the invention.

Cross-sectional structures of the above samples were observed andanalyzed by using an FE-SEM (field emission scanning electronmicroscope) including an EBSD (electron backscatter diffraction)analysis system and an image analysis method as described above.

Table 1 shows analysis results of Examples 1 to 15 and ComparativeExamples 21 to 27. In a column “Average grain size” in the Table 1, acolumn “D1 (interface grains) (μm)” shows the average grain size D1 ofinterface grains, and a column “D2 (non-interface grains) (μm)” showsthe average grain size D2 of non-interface grains. Furthermore, a column“D1/D2 (%)” shows a ratio (D1/D2) in % of the average grain size D1 ofthe interface grains to the average grain size D2 of the non-interfacegrains. Also, an area ratio (S0/S2) of the total area S0 of theinternal-crystal-inclusive grains to the area S2 of the non-interfacegrain group is shown in the column “S0/S2 (%)” by percent.

In the column “Thickness” in Table 1, the column “Sliding layerthickness T (mm)” shows the thickness T of the sliding layer, and acolumn “T1/T (%)” shows a ratio (T1/T) in % of the average thickness T1of an interface grain group to the thickness T of the sliding layer.

TABLE 1 Average grain size Thickness D1 D2 Interface Sliding Presence of(interface (non-interface grain group layer shear failure Sample Copperalloy grains) grains) D1/D2 S0/S2 thickness ratio Thickness ConditionsNo. Composition (mass %) (μm) (μm) (%) (%) (T1/T) (%) T (mm) A 1Cu—0.5Sn 35.2 298.2 12 43 8.7 0.35 None 2 Cu—6Sn 40.5 232.3 17 51 2.81.25 None 3 Cu—6Sn 46.0 239.3 19 55 3.1 1.2 None 4 Cu—8Sn 62.5 298.4 2161 5.3 1.0 None 5 Cu—10Sn 68.2 263.0 26 68 7.9 0.8 None 6 Cu—10Sn 73.3311.3 24 80 3.4 2.0 None 7 Cu—10Sn 35.8 298.3 12 25 7.7 0.4 None 8Cu—12Sn 79.0 269.1 29 52 2.6 3.0 None 9 Cu—6Sn—10Ni—1Fe—0.1P 68.1 288.324 49 4.8 0.8 None 10 Cu—6Sn—1Al—11n—3Ag—1Sb 72.3 284.4 25 47 5.2 0.8None 11 Cu—6Sn—10Zn—2Si—3Mn—15Pb 73.4 290.5 25 48 4.7 0.8 None 12Cu—10Sn—10Bi—1Fe—0.02P 70.2 296.2 24 46 4.9 0.8 None 13 Cu—6Sn 31.0227.3 14 53 2.8 1.25 None 14 Cu—6Sn 40.5 232.3 17 26 2.8 1.25 None 15Cu—10Sn 69.5 306.3 23 78 2.8 1.25 None 21 Cu—6Sn 0.8 (26.8) — — — 0.8Present 22 Cu—6Sn 29.1 316.3 9 68 3.4 0.8 Present 23 Cu—10Sn 28.8 306.49 45 3.1 0.8 Present 24 Cu—10Sn 84.9 258.8 33 70 10.6  0.8 Present 25Cu—6Sn 35.5 227.3 16 20 2.8 1.25 None 26 Cu—6Sn 28.0 215.3 13 0 2.5 1.05Present 27 Cu—10Sn—10Bi—1Fe—0.02P 53.5 54.4 98 0 — 0.8 Present SlidingManufacturing conditions Presence of Surface Amount of Molten InitialEnd shear failure Cracking poured copper alloy Back metal gravitygravity Sample Conditions Conditions copper temperature temperatureratio ratio No. B B alloy (° C.) (° C.) (F/Gi) (F/Ge) 1 Present None 10+50 −50 0.85 45 Examples 2 Present None 15 +50 −25 0.83 65 3 None None15 +75 −25 0.90 56 4 None None 15 +100 −25 0.88 59 5 None None 15 +100 00.85 63 6 None None 15 +100 −25 0.84 40 7 None None 10 +75 −50 0.87 70 8Present None 25 +150 −25 0.88 55 9 None None 20 +100 −25 0.84 64 10 NoneNone 20 +100 −25 0.85 48 11 None None 20 +100 −25 0.88 49 12 None None20 +100 −25 0.85 50 13 Present None 15 +50 −25 0.80 60 14 Present None15 +50 −25 0.86 70 15 Present None 15 +50 −25 0.90 40 21 — Present 2+100 −25 0.85 67 Comparative 22 — Present 7.5 +100 −25 0.84 62 Examples23 — Present 15 +100 −75 0.87 49 24 — Present 15 +100 +25 0.88 55 25Present Present 15 +50 −25 0.88 80 26 Present Present 15 +50 −25 0.98 6927 — Present — — — — —

The samples of Examples and Comparative Examples were subjected to asliding test with use of a bearing tester under conditions (A) as shownin Table 2. Furthermore, the samples of Examples were also subjected toa sliding test with use of the bearing tester under conditions (B) asshown in Table 3, in which a larger load was applied to the slidinglayer.

The samples of Examples and Comparative Examples after the sliding testswere cut in a direction parallel to a circumferential direction andperpendicular to a sliding surface, and observed of the generation of“shear failure” at an interface between the sliding layer and theback-metal layer with use of an optical microscope. In a column“Presence of shear failure” in Table 1, in a column “Conditions A” and acolumn “Conditions B”, “Present” indicates a case where the “shearfailure” was observed at the interface between the sliding layer and theback-metal layer of the sliding member after the sliding tests underconditions (A) and (B), and “None” indicates a case where no “shearfailure ” was observed at the interface after the sliding tests.

Furthermore, the samples of Examples and Comparative Examples after thesliding tests were visually observed for the occurrence of “cracking” onthe sliding surface. In a column “Sliding surface cracking” in Table 1,in a column “Conditions B”, “Present” indicates a case where “cracking”was observed on the sliding surface after the sliding test underconditions (B), and “None” indicates a case where no “cracking” wasobserved on the sliding surface after the sliding test.

TABLE 2 Conditions (A) Tester Bearing tester Load 25 MPa Rotationalspeed 600 rpm Operation mode Repetition of sliding for 0.5 minutes andstop for 5 minutes The number of cycles 20 times Lubrication oil VG46Oil supply amount 80 cc/min Oil temperature 50° C. Counter shaft SUJ2Counter shaft roughness 0.1 Ra

TABLE 3 Conditions (B) Tester Bearing tester Load 35 MPa Rotationalspeed 600 rpm Operation mode Repetition of sliding for 0.5 minutes andstop for 5 minutes The number of cycles 20 times Lubrication oil VG46Oil supply amount 80 cc/min Oil temperature 50° C. Counter shaft SUJ2Counter shaft roughness 0.1 Ra

As shown in Table 1 of the sliding test under conditions (A),Comparative Examples 21 to 24 showed “shear failure” occurred at theinterface between the sliding layer and the back-metal layer. On theother hand, in Examples 1 to 15, did not show “shear failure” at theinterface. This is presumably because, as described above, thecross-sectional structures of Examples 1 to 15 perpendicular to thesliding surface of the sliding layer have the average grain size D1 ofthe interface grains was 30 to 80 μm and was 10 to 30% of the averagegrain size D2 of the non-interface grains (D1/D2=0.1 to 0.3), and thusthe occurrence of shear failure was prevented by the effect alreadydescribed.

Furthermore, also after the sliding test under conditions (B) in which alarger load was applied to the sliding layer of the sliding member thanthe conditions (A), Examples 3 to 7 and 9 to 12 in which the averagegrain size D1 of the interface grains was in the range of 40 to 80 μmand the average thickness (average length from the bonding surfacetoward the sliding surface) T1 of the interface grain group was in therange of 3 to 8% of the thickness T of the sliding layer (T1/T=0.03 to0.08), did not show “shear failure” at the interface between the slidinglayer and the back-metal layer due to further enhancement of the aboveeffect.

In the structures of Examples 3 to 7 and 9 to 12, the amount of moltencopper alloy poured onto the inner surface of the cylindrical memberwas, as shown in Table 1, in the range of approximately 15 to 20 timesthe amount required to form a sliding layer having the thickness T, andthe temperature of the molten copper alloy during the pouring of themolten copper alloy was higher by approximately 75 to 100° C. than theliquidus temperature of the copper alloy.

After the sliding test under conditions (B), no “cracking” was observedon the sliding surface after the sliding test in the samples having thearea ratio (S0/S2) being 25 to 80%.

As shown in Table 1, Examples 1 to 15 did not show “cracking” occurredon the sliding surface in the sliding test under conditions (B). On theother hand, Comparative Examples 21 to 27 showed “cracking” on thesliding surface.

The sample of Comparative Example 21 was produced by a conventionalgeneral centrifugal casting method, and the amount of poured moltencopper alloy was set to a value corresponding to twice the thickness Tof the sliding layer. Thus, as described above, the molten copper alloycoming into contact with the bonding interface of the back-metal layerwas rapidly cooled, and a thin film of chill crystals having anexcessively small grain size (an average grain size of 0.8 μm) wasformed on the bonding interface of the back-metal layer. Therefore, inComparative Example 21, “shear failure” was observed at the interfacebetween the sliding layer and the back-metal layer of the sliding memberafter the sliding test under test conditions (A). In the cross-sectionalstructure of Comparative Example 21, chill crystals which are not incontact with the bonding surface of the back-metal layer were alsoformed above the copper alloy grain group in contact with the bondingsurface of the back-metal layer. In the column “Average grain size” inTable 1, the value in parentheses shown in the column “D2 (non-interfacegrains) (μm)” indicates an average grain size of grains (grains of acolumnar crystal portion above a chill crystal portion and small grainsof an equiaxed crystal portion above the columnar crystal portion)except the chill crystals which are not in contact with the bondingsurface of the back-metal layer.

In Comparative Example 22, the amount of poured molten copper alloy wasset to a value corresponding to 7.5 times the thickness T of the slidinglayer. In Comparative Example 23, the amount of poured molten copperalloy was set to a value corresponding to 15 times the thickness T ofthe sliding layer, and the temperature of the back-metal layer duringthe pouring of the molten copper alloy was lower by 75° C. than thetemperature of the molten copper alloy. Accordingly, the molten copperalloy coming into contact with the bonding interface of the back-metallayer during the pouring of the molten copper alloy was cooled fasterthan in Examples, leading to insufficient prevention of the simultaneousgeneration of a large number of crystal nuclei in the vicinity of thebonding interface of the back-metal layer and the simultaneoussolidification of the molten copper alloy in the vicinity of theinterface. The average grain size D1 of the interface grains was lessthan 30 μm, and due to excessively low ductility, “shear failure” wasobserved at the interface between the sliding layer and the back-metallayer of the sliding member after the sliding test under conditions (A).Furthermore, the average grain size D1 of the interface grains was lessthan 10% of the average grain size D2 of the non-interface grains. Dueto an excessively large difference in deformation resistance between theaverage grain sizes D1 and D2, shear failure (cracking) was alsoobserved in the vicinity of an interface between the interface graingroup and the non-interface grain group.

In Comparative Example 24, the temperature of the back-metal layer washigher by 25° C. than the temperature of the molten copper alloy duringthe pouring of the molten copper alloy. Thus, the molten copper alloycoming into contact with the bonding interface of the back-metal layerduring the pouring of the molten copper alloy was cooled slower than inExamples. The average grain size D1 of the interface grains exceeded 80μm, and thus deformation resistance of the interface grain group wasexcessively low. Furthermore, the average grain size D1 of the interfacegrains exceeded 30% of the average grain size D2 of the non-interfacegrains, and thus a difference in deformation resistance between theaverage grain sizes D1 and D2 was also excessively small. Therefore,“shear failure” was observed at the interface between the sliding layerand the back-metal layer of the sliding member after the sliding testunder conditions (A).

In Comparative Example 25, the average grain size D1 of the interfacegrains was 30 to 80 μm and was 10 to 30% of the average grain size D2 ofthe non-interface grains (D1/D2=0.1 to 0.3) in the cross-sectionalstructure perpendicular to the sliding surface of the sliding layer.Thus, no “shear” occurred at the interface between the sliding layer andthe back-metal layer of the sliding member after the sliding test.However, the ratio (S0/S2) of the total area S0 of theinternal-crystal-inclusive grains to the total area S2 of thenon-interface grains was less than 25%. Thus, elastic deformation of thenon-interface grain group was not prevented, and cracking occurred onthe sliding surface after the sliding test under conditions (B).

In Comparative Example 26, the temperature of the molten copper alloyand the temperature of the back-metal layer during the pouring of themolten copper alloy were appropriate. However, the initial gravity ratioF/Gi was not less than 0.9, leading to insufficient prevention of thesimultaneous generation of a large number of crystal nuclei in thevicinity of the bonding interface of the back-metal layer and thesimultaneous solidification of the molten copper alloy in the vicinityof the interface. The average grain size D1 of the interface grains wasless than 30 μm, and “shear failure ” was observed, due to excessivelylow ductility, at the interface between the sliding layer and theback-metal layer of the sliding member after the sliding test underconditions (A).

In Comparative Example27, the sample was produced by a conventionalgeneral sintering method. The average grain size D1 of the interfacegrains was approximately the same as the average grain size D2 of thenon-interface grains, and D1 was 98.3% of D2. Thus, there was almost nodifference in deformation resistance between the average grain sizes D1and D2. Therefore, “shear failure” was observed at the interface betweenthe sliding layer and the back-metal layer of the sliding member afterthe sliding test under conditions (A).

1. A sliding member comprising: a back-metal layer having a back surfaceand a bonding surface; and a sliding layer on the bonding surface of theback-metal layer, the sliding layer having a sliding surface, whereinthe back-metal layer comprises an Fe alloy, wherein the sliding layercomprises a copper alloy comprising 0.5 to 12 mass % of Sn and thebalance of Cu and inevitable impurities, wherein the sliding member hasa cross-sectional structure perpendicular to the sliding surface, thecross-sectional structure comprising first copper alloy grains that arein contact with the bonding surface and second copper alloy grains thatare not in contact with the bonding surface, wherein the first copperalloy grains has an average grain size D1 and the second copper alloygrains has an average grain size D2, and D1 and D2 satisfy the followingrelations: D1 is 30 to 80 μm; and D1/D2=0.1 to 0.3, wherein, in thecross-sectional structure, the second copper alloy grains comprise thirdgrains that includes internal grains therein that are not in contactwith a grain boundary of the third grains, and wherein a total area S1of the third grains and a total area S2 of the second copper alloygrains satisfy the following relation: S0/S2=0.25 to 0.80.
 2. Thesliding member according to claim 1, wherein the second copper alloygrains are columnar crystals.
 3. The sliding member according to claim1, wherein the average grain size D1 of the first copper alloy grains is40 to 80 μm.
 4. The sliding member according to claim 1, wherein anaverage thickness T1 of a group of the first copper alloy grains is 3 to8% of a thickness T of the sliding layer.
 5. The sliding memberaccording to claim 1, wherein a thickness T of the sliding layer is 0.4to 2.0 mm.
 6. The sliding member according to claim 1, wherein thecopper alloy further includes one or more elements selected from 0.01 to0.2 mass % of P, 0.1 to 15 mass % of Ni, 0.5 to 10 mass % of Fe, 0.01 to5 mass % of Al, 0.01 to 5 mass % of Si, 0.1 to 5 mass % of Mn, 0.1 to 10mass % of Zn, 0.1 to 5 mass % of Sb, 0.1 to 5 mass % of In, 0.1 to 5mass % of Ag, 0.5 to 25 mass % of Pb, and 0.5 to 20 mass % of Bi.
 7. Thesliding member according to claim 1, wherein the back-metal layer has acomposition including 0.07 to 0.35 mass % of C, not more than 0.4 mass %of Si, not more than 1 mass % of Mn, not more than 0.04 mass % of P, notmore than 0.05 mass % of S, and the balance of Fe and inevitableimpurities.